1. Field of the Invention
The invention concerns a method to adapt the width and position of the central value of a greyscale windowing for imaging with an x-ray device, in particular with a computed tomography apparatus, based on CT values determined with the x-ray device. The invention also concerns an x-ray device, a computer, and a non-transitory computer readable data medium that implement such a method.
2. Description of the Prior Art
In medical engineering, in imaging with x-ray radiation, for example in computed tomography, it is always sought to apply an optimally low dose of x-ray radiation to the patient for the generation of one or more images of a tissue of the patient. A defined image quality must also be achieved, however, in order to be able to solve a clinical problem using the generated image or images, which requires a minimum dose. Relevant measures for the image quality are the image noise or the image contrast, for example. In particular, the image contrast of specific elements and materials, such as the iodine used as contrast agent in computed tomography, has a relatively strong dependency on the spectrum of the x-ray radiation that is used. Given use of relatively lower tube voltages, the spectrum of the x-ray radiation is such that the image contrast of iodine increases. Given the use of iodine, the dose of x-ray radiation that is applied to a patient to achieve an optimally high image quality depends on the spectrum of the x-ray radiation.
In CT angiography to show blood-carrying vessels, in which the visibility of iodine is of practically exclusive importance, the dose of x-ray radiation applied to the patient is therefore reduced by the use of relatively lower tube voltages (see M. J. Siegel et al., “Radiation Dose and Image Quality in Pediatric CT: Effect of Technical Factors and Phantom Size and Shape”, Radiology 2004; 233: Pages 515 to 522).
In “Automatic selection of tube potential for radiation dose reduction in CT: A general strategy”, L. Yu et al. propose a method to select a tube voltage suitable for a defined examination with regard to a reduction of the dose of x-ray radiation, in which an iodine contrast-to-noise ratio is used as an image quality index in connection with a noise condition a in order to be able to characterize and account for the different requirements for the image quality for different examinations. Different noise conditions a are provided for different examinations. Such a noise condition should result in the absolute image noise does not exceed a certain value. For a CT angiography in which the iodine contrast-to-noise ratio is of practically exclusive relevance, the noise condition a is chosen between 1.5 and 2.0. For breast, back or pelvic examinations with contrast agent, the noise condition a is selected between 1.1 and 1.25, and for breast, back or pelvic examinations without contrast agent the noise condition α is set equal to one. Based on a “relative dose factor” (RDF) (in which the contrast-to-noise ratio of iodine as well as the noise condition a are taken into account), the tube voltage is determined with which the lowest dose of x-ray radiation is applied to the patient for a specific examination.
In DE 10 2010 043 712.3 (not published prior to the filing date to which the present application is entitled), a method is described for the determination of the value of a tube voltage of an x-ray tube of an x-ray device to generate at least one image of a specific tissue of a patient that is to be examined. In this method the value of the tube voltage is determined such that upon adjustment of the value of the tube voltage at the x-ray tube and the acquisition of at least one x-ray projection of the defined tissue of the patient that is to be examined for generation of the at least one image of said defined tissue of the patient that is to be examined, the dose of x-ray radiation applied to the patient is as low as possible with consistent image quality.
The method proposed in DE 10 2010 043 712.3 (also not published prior to the filing date to which the present application is entitled) is based on the consideration that, for virtually every examination of a tissue, given a consistent image quality a tube voltage can be found at which the dose of x-ray radiation to be applied to the patient for the examination of said tissue can be reduced or optimized. The optimization thus depends on the tissue to be examined, which is to be shown in at least one image.
The starting point of this method is the consideration of the dependency of the contrast of the defined tissue to be examined (and possibly the dependency of the contrast of the defined tissue to be examined that is provided with a contrast agent) on the spectrum of the x-ray radiation of the x-ray tube, or on the value of the tube voltage that is relevant to the generated spectrum of the x-ray radiation. Furthermore, an image quality required or desired for a diagnosis, or solution to a clinical question that is intended using the at least one generated image of the defined tissue to be examined, is predetermined by at least one parameter establishing or describing the desired image quality, which can be a reference tube voltage and a reference tube current, for example, or a reference noise. Under consideration of this at least one parameter establishing or describing the image quality, and given a constantly maintained contrast-to-noise ratio, the tube voltage is determined as an image quality value, for which the dose of x-ray radiation applied to the patient is lowest in the acquisition of one or more x-ray projections to generate at least one image of the defined tissue of the patient that is to be examined.
A greyscale windowing to show the CT values based on the acquisition of x-ray projections, which CT values represent the defined tissue and tissue surrounding the defined tissue, is associated with the reference tube voltage, in which specific grey values are assigned to specific CT values. If the reference tube voltage is not used for the acquisition of the x-ray projections but rather that tube voltage at which the dose of x-ray radiation applied to the patient is lowest for the imaging of the defined tissue of the patient that is to be examined, the maintenance of the greyscale windowing that is associated with the reference tube voltage can lead to the situation that the image impression (human visual appearance) is perceived as unacceptable due to the changes of the CT values that are caused by the set tube voltage.